Non-color-dependent vignetting is a photographic phenomenon in which the picture taken by the camera exhibits a darkening from the center to the corners irrespective of what color is present. Photographs taken by both a film camera and images taken by a digital camera can exhibit vignetting. Some amount of vignetting is acceptable, and actually desirable from an artistic sense. But an excessive amount of vignetting will be objectionable to most people.
In either a film camera or a digital camera, such vignetting is caused by having to capture an image on a small flat surface, namely the film or the optical-to-electronic imager, e.g., a charge coupled device (“CCD”). In either type of camera, rays of light striking the corners of the film/imager travel a longer path and arrive at a different angle of incidence than rays of light that impinge directly upon the center of the film/imager.
Differences in the response of the film/imager to light impinging directly versus light impinging at an angle produce non-uniformity in the captured image, some of which is characterized as vignetting. The lens system can also effect vignetting. Plus, if the camera uses a flash device, vignetting can also be due to the flash device's variation in illumination intensity across the subject.
The image sensor of a digital camera can introduce color-dependent vignetting as well. A typical color image sensor in a digital camera includes a mosaic type of image sensor, e.g., a CCD, over which is formed a filter array that includes the colors red, green and blue. Each pixel has a corresponding red, green or blue filter area. A typical implementation of the color filter repeats a two-line pattern. The first line contains alternating red and green pixels and the second line contains alternating blue and green pixels. The larger number of green pixels represents an adaptation for the increased sensitivity to green light exhibited by the human eye. The separate color arrays, or planes, of data images formed by the sensor are then combined to create a full-color image after suitable processing.
Color-dependent vignetting should be understood as color-sensitivity variation exhibited in the image captured by the image sensor from the center of the image to the corners. This can be exhibited in addition to regular non-color-dependent vignetting. The human eye is very sensitive to color-dependent variation. Hence, color-dependent vignetting is very objectionable, even in small amounts.
Color-dependent vignetting can be minimized by using higher quality lens systems, image sensors and/or flash devices. But this is an impractical solution in a situation in which minimization of system cost and/or system complexity are important design criteria.